The present invention relates to a composition comprising a refractory substrate having a first coating layer of a calcium component and a second coating layer of an alumina component, and, more particularly, to a method for improving the strength of a ceramic composite by preparing an alpha alumina substrate having deposited thereon a first coating of calcium pyrophosphate and a second coating of hydrous alumina. Whereupon calcining said composition, said second coating converts to an anhydrous crystalline form of alumina.
Ceramic composites start with a ceramic preform which is usually based on silicon carbide, silicon nitride, aluminum silicate, aluminum oxide or mixtures thereof. These preforms are imbedded in a ceramic matrix to form composites.
Ceramic composites have been found to lose their strength at higher temperatures; a problem which has not yet been resolved. All the useful properties begin in the early stages of ceramic processing. The structural integrity of each material must be preserved through a variety of thermal and mechanical stress exposures. Thus, the need for a high-temperature, high-strength material exists.
The unique properties of the present invention enable the composition to be used as a preform for a ceramic matrix composite having unexpected strength and toughness. These unexpected results are attributed to the coating that provides a debonding function between the preform and the matrix which results in limiting crack propagation in the composite body.